This invention relates generally to plasma ashing systems. More specifically, the invention relates to a method for detecting an endpoint for an oxygen free plasma process used for removing photoresist and/or residues from a substrate. In particular, the oxygen free plasma reacts with photoresist and/or residues to produce optical emission signals from the reaction products which are then optically monitored for determining the ashing endpoint.
Ashing is a plasma process by which photoresist and residues are stripped or removed from a semiconductor wafer or the like upon exposure to a plasma. The ashing process generally occurs after an etching, implantation or deposition process has been performed in which a photoresist material is used as a mask for etching a pattern or depositing ions into the underlying substrate. The photoresist and residues remaining on the wafer after the etch or deposition process need to be removed. Typically, the plasma used for the ashing process is generated from a gas mixture containing oxygen as a component. The highly reactive oxygen containing plasma removes photoresist and residues by an oxidization reaction. The oxidation or combustion products resulting from the ashing operation are typically volatile components such as carbon dioxide and water vapor, and are carried away in a gas stream.
A problem with ashing is the accuracy in determining when the photoresist and/or residues have been removed. Accurate end point detection is critical to a high yield of high performance integrated circuits. Underetching or overetching is generally undesirable for numerous reasons well known to those skilled in the art. For instance, overetching causes linewidth variations which can affect device performance since the tolerance of the critical dimensions of the patterned circuit is very small.
One such technique in determining when the photoresist and/or residues have been removed is by in situ monitoring of the reaction between the photoresist and the plasma. This can be accomplished by a variety of means such as by optical emission spectroscopy, mass spectrometry, laser interferometry, ellipsometry and other techniques generally known to those skilled in the arts. Of these, optical emission spectroscopy is most preferred due to the non-intrusiveness, relative inexpense and durability compared to the other monitoring techniques. Many endpoint detection schemes based on optical emission spectroscopy have been defined for oxygen containing plasma ashing processes by monitoring particular spectral lines or regions determined by either a chemical constituent in the plasma and/or the emission signals produced by the reaction between the plasma and the photoresist and/or residues. The OH or CO functionalities are predominantly monitored since these are the primary emission signals produced during the oxygen containing plasma stripping process of photoresist and/or residues.
A problem that has surfaced recently with oxygen containing plasmas is that they are generally unsuitable for use with copper and most low k interconnects. Copper and low k materials are preferred for many reasons as device manufacturers transition beyond the 0.35 xcexcm design rules. For example, copper has lower resistivity than aluminum which allows it to carry more current in a smaller area, thus enabling faster and denser integrated circuits or the like with increased computing power. Moreover, new insulating materials for interconnects, such as low k dielectrics, result in lower interconnect capacitance and crosstalk noise, thereby enhancing circuit performance. Low k dielectrics can generally be defined as those materials suitable for use in the manufacture of integrated circuits, or the like, having dielectric constants less than about 3.5. These low k dielectrics can be broadly grouped into four categories: organic, doped oxides, porous and TEFLON-like. Most low k dielectrics do not tolerate the presence of oxygen, especially in the form of plasma. It is well known that many of the low k dielectrics in the aforementioned categories are either etched at roughly the same rate as photoresist, or show better etch selectivity but suffer an increase in k value during exposure to oxygen containing plasmas. Even using very dilute oxygen mixtures, often used to slow the resist strip rate, has not overcome this problem for many of the low k materials.
Accordingly, the use of oxygen free plasmas have been found effective at removing photoresist and/or residues from low k materials. One such oxygen free plasma process for photoresist stripping is disclosed in the U.S. patent application Ser. No. 09/368,553 filed Aug. 5, 1999, entitled xe2x80x9cOxygen Free Plasma Stripping Process,xe2x80x9d which is incorporated herein by reference in its entirety. Optical methods for detecting the end point using the oxygen free plasmas for stripping photoresist and/or residues have yet to be defined. The existing methods of detecting the ashing endpoint are not adequate for detecting an endpoint during the use of the oxygen free plasma since the OH or CO functionalities are not sufficiently present to generate a signal having the desired magnitude for accurate endpoint detection.
Thus, there is a need for a method for detecting an ashing endpoint for an oxygen free plasma photoresist and/or residue removal process.
One object of the present invention is to provide a robust method for accurately determining the ashing endpoint for an oxygen free plasma stripping process. The inventive method, by precisely determining the ashing endpoint, advantageously avoids the problems associated with overetching or underetching. Moreover, the inventive method monitors a wavelength or a wavelength range having an emission signal that has a much higher signal to noise ratio than that previously observed from measuring the conventional transitions of OH or CO. As such, the ashing endpoint can be used in an intrinsically noisy system that includes, but is not limited to, radiation emitted from wafer heating lamps and radiation emitted from the plasma itself
The present invention comprises the steps of placing a substrate having the photoresist and/or residues thereon into a reaction chamber. A gas composition containing a nitrogen gas and a selected one of a hydrogen bearing gas, a fluorine bearing gas and a fluorine-hydrogen bearing gas mixture is excited to form an oxygen free plasma. The oxygen free plasma reacts with the substrate having the photoresist and/or residues thereon to produce emitted light signals corresponding to reaction byproducts having unconventional transitions other than CO or OH. The light emission intensity signals resulting from the reaction byproducts are sequentially recorded over a period of time. The endpoint is determined at a time when the light emission intensity signals of the reaction product are no longer detectable. The reaction produces, among others, oxygen free compounds or oxygen free radicals which emit at a primary signal at a wavelength of about 387 nm and secondary emission signals at about 358 nm and at about 431 nm.
In one embodiment, emitted light within a wavelength range that includes the primary emission signal at about 387 nm is recorded by a spectrometer, such as by a CCD based spectrometer. When the intensity of the emission signal at about 387 nm within that wavelength range is no longer detectable or reaches a steady state below a threshold value, the ashing endpoint is determined by an appropriate algorithm and the plasma is turned off to prevent overetching. Preferably, the wavelength range includes the secondary emission signals at about 358 nm and about 431 nm. The predetermined threshold value represents contributions to the emission signal at primary and secondary wavelengths that are not caused by the reaction between the photoresist, residues and plasma.
In another embodiment, a specific wavelength of about 387 nm is optically measured, such as by a monochromator. A first emission intensity signal at the primary emission signal of about 387 nm is measured prior to reacting the plasma with the photoresist and/or residues. The first emission intensity signal represents the background radiation which comprises radiation from sources such as the plasma, and the wafer heating lamps. Preferably, a blank or dummy wafer is run during measurement of the first intensity signal. The substrate having photoresist and/or residues thereon is then exposed to reactive species of the plasma to generate a second emission intensity signal at about 387 nm. Preferably, the reactive species of the plasma are electrically neutral. The photoresist and/or residues react(s) with the reactive species of the oxygen free plasma to produce unconventional volatile products, products that are very different from those produced during exposure to an oxygen containing plasma. These products emit the primary light signal at about 387 nm and secondary light signals at about 358 nm and about 431 nm. The method of detecting an ashing endpoint further involves measuring the second emission intensity signal at the primary light signal of about 387 nm as the plasma is exposed to the substrate. Alternatively, the secondary light signals can be measured for determining endpoint. The ashing endpoint of the plasma stripping process is determined by comparing the first intensity signal with the second intensity signal wherein the endpoint is detected when said first and second intensities are about the same. Consequently, the ashing endpoint is determined when the reaction products are no longer being evolved, an indication that the photoresist and/or residues are no longer present in sufficient quantities to produce a detectable emission signal.
The present invention can be used to determine the ashing endpoint for any oxygen free plasma process wherein the conventional transitions of CO and OH are not sufficiently present to generate a detectable light signal having the desired magnitude for accurate endpoint detection. The particular components of the gas composition are selected by their ability to form a gas and a plasma at plasma forming conditions, and often by the lack of damage to low k materials. Preferably, the gas composition for generating the oxygen free plasma contains nitrogen gas and a reactive gas. Preferably, the reactive gases are a selected one of a hydrogen bearing gas, a fluorine bearing gas and a fluorine-hydrogen bearing gas mixture to form the oxygen free plasma. In cases where no nitrogen is present as a mixture of any of the reactive gases, i.e. the fluorine bearing compound or the hydrogen bearing compound, nitrogen is added separately as a process gas. Preferably, the components are combined and added to the plasma asher as a gas.
Preferably, the methods for optically measuring the emission signals during reaction of the plasma with the photoresist and/or residues is by the use of a light detector. Equipment suitable for use in present invention include a monochromator, a spectrometer, or the like. Other spectroscopic methods suitable for use in the present invention will become apparent to those skilled in the art in view of this disclosure. The invention is not intended to be limited to any particular optical emission spectroscopy means or filters. Generally, the differences between the various spectrometers is the range of wavelength monitored and the differentiation capability for distinguishing background radiation from the emitted species of interest. It is well within the skill of those in the art to determine how the various spectrometer configurations or the like can be used to differentiate the background radiation from the radiation emitted from the reaction of the plasma, photoresist and residues.
These and other objects, advantages and features of the invention will become better understood from the detailed description of the invention which is described in conjunction with the accompanying drawings.